INTRODUCTION. 
xi 
laws, gives a result very wide of the truth, unless the various disturbing influences, as 
refraction, parallax, precession, nutation, and all the numerous sources of error are accurately 
judged and allowed for. In like manner, if we really knew the nature and value of the 
disturbing influences acting in concert with the law of generation, we should be able to 
calculate with equal accuracy the problems presented in the phenomena of life. 
These remarks are made in no carping spirit, but in the conviction that an evolution theory 
is not alone sufficient to fulfil the conditions of the case, that the limit to which evolution 
exists and the causes of limitation are unknown. If it be the correct solution to the enigma 
if it be the truth — the pointing out of difficulties can do no damage to its cause : it may 
stimulate discussion and inquiry ; but light and truth go hand in hand. The truth is like a 
mighty torrent — now in view, now lost to sight in deep gorges — that only flows on with 
more resistless force for the obstruction of a temporary barrier, and, ever expanding, ever 
inci easing in power, loses itself at last in eternity. Magna est veritas et prcevalebit. 
But fuither, if the capability of variation in direct descent be so wide and so universally 
in action as the theory of modified descent requires us to believe, classification on a natural 
sj stem is simply impossible with regard to time ; for we know not, and can never hope to 
know to any appreciable extent, the lineage of even a single genus, the blanks in the 
geologic record preventing the recognition of progenitors in their altered posterity; and as 
regards space, to which indeed almost all classification is limited, any system of arrangement 
based on actual affinity is equally hopeless; for the degree of relationship by no means 
varies as the similarity either in form, habit, or structure ; and if the lineage be unknown 
the former can not be determined with certainty by the latter. If it could be so determined, 
o course a close study of the structure and habits of any particular individual would fix 
positively its nearest relations and exact place in the « schema naturse.” Among individuals 
of a species it often happens that the most nearly related exhibit wide differences, while the 
istantly related show strange resemblances, which would entirely mislead a classifier 
unacquainted with their origin and descent; and if we assume that different species as well 
as different individuals may be descended from a common stock, we must also take for 
granted that the same phenomena will occur; and in this case there will be wide differences 
between closely related species, and great similarity sometimes displayed between those 
whose connexion is more remote. In short, resemblances in structure and habit do not 
vary directly as the amount of consanguinity, and, though of great value in any classi- 
fication, they do not suffice alone to fix the position of a species or genus according to a 
natural theory. 
Let us imagine for a moment a philosopher from another world attempting to classify the 
diffei ent varieties of the human race, which most people agree in considering to be descended 
fiom a single original stock; and though he would probably easily define the broad lines of 
division, when lie reached the smaller groups, the difficulties that he would encounter 
become evident in the aberrant nature of the variations in individual size, shape, and colour, 
so general among mixed races. 
Cases of somewhat analogous arrangements among birds will occur to the mind of every 
ornithologist ; and a few minutes’ reflection will convince the reader that to subdivide a 
b 2 
